Disable fielddata use for analyzed strings

By default, users should not be sorting, aggregating, or scripting against the analyzed string variant of fields. Instead, users should do those things against the `.raw` multifield variant.

This _will_ prevent the use of `significant_terms` aggregation against the associated field, which will be undesirable in rare cases. Given the rarity of that need, I would much rather have those users manually enable fielddata explicitly for the field (it is dynamically updatable!).

Fixes #309

Author: pickypg

Diff for: lib/logstash/outputs/elasticsearch/elasticsearch-template.json

@@ -11,7 +11,8 @@
           "match" : "message",
           "match_mapping_type" : "string",
           "mapping" : {
-            "type" : "string", "index" : "analyzed", "omit_norms" : true
+            "type" : "string", "index" : "analyzed", "omit_norms" : true,
+            "fielddata" : { "format" : "disabled" }
           }
         }
       }, {
@@ -20,6 +21,7 @@
           "match_mapping_type" : "string",
           "mapping" : {
             "type" : "string", "index" : "analyzed", "omit_norms" : true,
+            "fielddata" : { "format" : "disabled" },
             "fields" : {
               "raw" : {"type": "string", "index" : "not_analyzed", "doc_values" : true, "ignore_above" : 256}
             }